This invention relates in general to electro-optical devices and in particular to an electro-optical device with an inverted transparent substrate.
Light emitting diodes (LEDs) are frequently used for displays and indicators as well as fiberoptic sources, In one type of LED, a p-n junction semiconductor is employed. A potential difference is applied across the junction by means of a pair of electrode contacts in contact with the p-type and n-type regions. This causes electrons to be injected across the junction from the n-type region to the p-type region and causes holes to be injected across the junction from the p-type region to the n-type region. In the p-type region, the injected electrons recombine with the holes resulting in light emission; in the n-type region, the injected holes recombine with electrons resulting in light emission. The wavelength of the light emission depends on the energy generated by the recombination of electrons and holes, which is determined by the band gap of the p-n junction semiconductor material.
To enhance the efficiency of light emission, it is known to those skilled in the art to be preferable to cause only one of the two types of carriers, namely electrons or holes, to be injected across the junction, but not both. In improved LEDs, a p-n single heterojunction semiconductor is employed. A heterojunction is the interface between two different types of materials; thus, a heterojunction is formed at the junction between a p-type and an n-type semiconductor material. In a single heterojunction device, the energy band gap in the p-type region is different from that in the n-type region so that either electrons or holes, but not both, are injected across the junction The injected electrons or holes then recombine to cause light emission. For example, if the materials of the n-type and p-type regions are selected with the n-type region having a wider band gap than the p-type region, this has the effect of causing the electrons injected from the n-type region to the p-type region to have a lower potential barrier than the holes injected in the opposite direction. Thus, essentially only electrons will be injected across the junction, and the p-type layer where radiative recombination takes place is referred to below as the p-active layer.
A device known as a double heterojunction LED further improves on the efficiency of single heterojunction LEDs by the addition of another p-type layer of higher band gap material between the p-active layer and the substrate. A second heterojunction is thus effected between the p-active layer and the additional p-type layer. The higher band gap of the additional p-type layer helps to confine the injected electrons within the smaller band gap p-active layer. This allows for a much thinner p-active layer which minimizes re-absorption and increases light emission efficiency. Also the extra p-type layer provides another window for light out of the p-active layer.
The p- or n-type layers of various band gaps are typically grown as epitaxial layers from the alloys of III-V compounds. For example, an efficient red LED can be fabricated from aluminum gallium arsenide (AlGaAs) semiconductor material. The energy band gap of the semiconductor material can be increased with substitution of aluminum atoms for gallium atoms. The greater the aluminum substitution in the resulting material, the higher is the band gap. The varying concentration of aluminum does not affect the lattice constant, and this property allows successive epitaxial layers of lattice -matched AlGaAs to be grown easily.
Typically, to minimize re-absorption, the band gaps of all layers are chosen to be wider than that of the active layer. In this way, these layers appear transparent to the red light emitted from the active layer. By the same consideration, the substrate on which the epitaxial layers are grown should ideally have a wider band gap. However, it is not possible to obtain AlGaAs in wafer form, and instead, the lattice-matched GaAs is commonly used as a substrate.
Gallium arsenide (GaAs) has a band-gap of 1.43 electron volts (ev) which corresponds to the infrared end of the light spectrum. It is therefore highly absorptive of, and opaque to, visible light. Thus, LED devices formed on GaAs substrates are inherently disadvantageous in that the emitted light in the solid angle subtended by the opaque substrate, as well as light reflected by critical angle reflection at the upper surface, is lost to absorption. This may amount to more than 80% of the total light output.
It is possible to have transparent substrate LEDs, an example of which is disclosed by Ishiguro, et al. in Applied Physics Letters, Vol. 43, No. 11, pages 1034 to 1036, Dec. 1, 1983. Ishiguro, et al. report an AlGaAs red LED with an efficiency of 8%. The LED reported has the advantage of a transparent substrate, but is much more difficult and costly to make. The process involves growing various transparent layers of AlGaAs on an absorbing GaAs substrate. The first transparent layer adjacent to the GaAs substrate serves as a substitute substrate and subsequent layers constitute the device. The opaque GaAs substrate is subsequently removed, leaving the device on the substitute transparent substrate only.
A transparent substrate LED device normally includes a transparent substrate and device layers whose electrical and optical properties are critical for the performance of the device. To provide support for the device, the substitute transparent substrate must be grown sufficiently thick while maintaining a sufficiently good surface for the epitaxial growth of the device layers. This is costly and time consuming for two reasons. First, for the substrate to be transparent, an AlGaAs composition with a high concentration of aluminum must be used. The surface of epitaxial growth from such a composition is prone to oxygen absorption, in which event further growth is either degraded or impeded. Secondly, imperfections will compound in growing a thick layer of AlGaAs, even under the best of conditions. This provides a much less than ideal substrate. Subsequent epitaxial growth of the fairly thin, critical device layers on a rough substrate will inevitably result in large sample-to-sample variations, if not high failure rates. The dislocation density in each subsequent layer inherently increases as compared with the earlier grown layers.